The present invention relates to a system for monitoring errors in a measuring device for measuring the relative position of two objects, of the type comprising a scale, a scanning unit which generates at least one measuring signal in response to the scanning of the scale, an evaluating unit which evaluates the measuring signal, and means for generating an error signal in response to selected parameters of the at least one measuring signal.
German Patent DE-PS No. 20 22 151 discloses a method for avoiding errors in an incremental measuring device for measuring the relative position of two objects. In the disclosed system, phase-displaced scanning signals obtained in the scanning of a scale by means of a scanning unit are fed simultaneously to an evaluating unit and to an error monitoring unit. This error monitoring unit monitors the phase angle and the amplitude level of the scanning signals. In the event of faulty signal parameters of the scanning signals, for example as a result of optical fouling, aging or wear of the measuring system, this error monitoring unit generates an error signal which is conducted via a separate line to a warning lamp, which can be provided in the evaluating-and-display unit.
In addition, it is a known practice in such an incremental measuring device for an error monitoring system to switch the scanning signals and their inverted signals into a high resistance state during the time of the disturbance as indicated by the error signal. In this way, the disturbance appears as a line break to the evaluating unit. This approach to error signal transmission has the advantage that no separate transfer line is required for the error signal. It presents this advantage that the recognition of the high-resistance state can occur only relatively slowly because of line capacitance, especially in the case of great lengths of the transmission lines. This is because no driver function is provided for the charge reversal.